1. Field of the Invention
The present invention relates generally to a thin film transistor liquid crystal display (TFT-LCD), and more particularly to a method of making a thin film transistor liquid crystal display fabricated through four photomask processes.
2. Description of the Related Art
A conventional thin film transistor liquid crystal display primarily includes a thin film transistor (TFT) array substrate, a color filter and a liquid crystal layer, wherein the TFT array substrate includes a transparent substrate with a plurality of scan lines and data lines. These scan lines and data lines are perpendicular to define a plurality of subpixels. In each subpixel, there is a thin film transistor, a storage capacitor and a pixel electrode. The thin film transistor is used to be a switch member of the liquid crystal display, which includes a gate, a gate insulating layer, a channel layer, an ohmic contact layer, a source/drain, a passivation layer and a pixel electrode layer. It needs five or six photomasks to make above structure layers in the conventional process of making the thin film transistor liquid crystal display.
For a process using five photomasks, the first photomask process is used to define a first metal layer to form the scan lines and the gate of the thin film transistor. The second photomask process is used to define the channel layer and the ohmic contact layer of the thin film transistor. The third photomask process is used to define a second metal layer to form the source/drain of the thin film transistor and the data lines. The fourth photomask process is used to pattern a passivation layer to form a via. The fifth photomask process is used to pattern a transparent conductive layer to form the pixel electrode. However, the more photomasks used in the process take the higher cost for the process and cannot simplify the process.
In additional, the conventional storage capacitor is formed by use of the patterned first metal layer as the bottom electrode and overlapped by the second metal layer as the top electrode. Due to the metal layer is opaque, the areas of storage capacitors in the subpixels are not pervious to light so that the aperture ratio is reduced.